Molecular biology and energetics of membrane transport.

Abstract

Bacterial cytoplasmic membrane vesicles have provided a unique model system for the study of active transport (Kaback, 1970, 1972, 1974b), and have led to the development of similar experimental systems from the cells of higher organisms (Murer and Hopfer, 1974; Colombini and Johnstone, 1974; Sigrist-Nelson et al., 1975; Quinlan et al., 1976; Lever, 1976a, 1976b). Vesicles are prepared by lysis of osmotically-sensitized cells (i.e., protoplasts or spheroplasts), and consist of osmotically-intact, unit-membrane bound sacs which are approximately 0.5 to 1.0 μ in diameter (Kaback, 1971; Short et al., 1975). The sacs are devoid of internal structure, their metabolic activities are restricted to those provided by the enzymes of the membrane itself, and a number of observations demonstrate that the vesicle membrane has the same orientation as the membrane in the intact cell (see Stroobant and Kaback, 1975, for a summary of these observations). Transport by membrane vesicles per se is practically nil, and the energy source for the transport of a particular substrate can be determined by studying which compounds or experimental manipulations drive solute accumulation. Moreover, metabolic conversion of the transport substrate and the energy source is minimal.